Encyclopedia of Psychology and Law

states. Indeed, this task, perhaps more than any other,

tends to be handled by court-appointed experts.

Commentary on the Probative

Value of Other Evidence

A large segment of psychological expertise is devoted

to the subject of how people ordinarily respond, men-

tally and behaviorally, to different sets of circum-

stances. For example, research on domestic violence

indicates that many victims of such abuse fail to leave

battering relationships for a variety of psychological

and sociological reasons. Similarly, research on victims

of rape and sexual assault indicates that many of them

do not report the crime immediately, again for a variety

of reasons. In the law, this kind of research might be

relevant on a couple of related issues. Specifically, it is

sometimes offered to buttress the credibility of a wit-

ness. For instance, this research might suggest that an

alleged victim of sexual assault is not an untruthful wit-

ness because he or she failed to report the crime in a

timely fashion, since it is not unusual for sexual assault

victims to delay reporting. This sort of testimony is also

proffered for the more general purpose of giving triers

of fact background information regarding the usual cir-

cumstances that surround similar situations. For exam-

ple, triers of fact might be informed in a case involving

a defendant who was battered that a large percentage of

battered women do not leave abusive situations. Courts’

receptivity to these uses varies.

In most jurisdictions, evidence cannot be intro-

duced for the specific purpose of buttressing the cred-

ibility of a witness. Courts consider the use of

evidence to support a witness’s credibility to be an

invasion of the province of the jury. Juries are

entrusted with the task of assessing credibility. At the

same time, however, many courts allow expert testi-

mony regarding how people tend to respond to partic-

ular situations for the purpose of educating the jury

regarding the background context confronted by the

testifying witness. Three basic contexts arise with

regard to this kind of evidence. In some cases, such as

sexual assault cases, psychological evidence is prof-

fered not to show that a witness is truthful but to

demonstrate that, in light of how other people respond

in similar situations, the witness’ account is not unbe-

lievable. In the second, the expert does not speak to

the credibility of a witness but is offered to inform the

jury regarding the likely accuracy of a witness’s testi-

mony. In the third group, experts are offered to speak

to credibility directly, usually through the use of a test

such as a polygraph. These three contexts will be con-

sidered in turn.

In many cases, especially including rape cases and

sexual assault of children, psychological expertise is

proffered to show that the alleged victim’s behavior is

not inconsistent with that of others who have experi-

enced similar trauma. Evidence, for example, that an

alleged rape victim failed to immediately report the

crime might be put into perspective by expert testi-

mony that many sexual assault victims behave simi-

larly. This can be important evidence in a case in

which the defendant claims consent because the wit-

ness’s behavior might otherwise appear inconsistent

with having been assaulted. In this view, evidence on

the reporting rates of sexual assault are not offered to

support the credibility of the witness but to inform the

jury about how people tend to respond to similar situ-

ations. As many courts and commentators have

observed, however, there is a very fine line—if any

line at all—between the permitted purpose of rebut-

ting the defendant’s contention that the victim’s

behavior is inconsistent with having been assaulted

and the prohibited purpose of supporting the prosecu-

tion’s contention that the victim’s behavior is consis-

tent with having been assaulted. It is illogical to

permit expert evidence to prove nonconsent but not to

prove that a rape occurred (i.e., in most cases, non-

consent). But most courts adhere to this distinction.

The second group of cases is not overly controver-

sial as regards invading the trier of fact’s province to

decide witness credibility, because it principally con-

cerns the general accuracy of similarly situated wit-

nesses. The lion’s share of this type involves expert

opinion on the reliability of eyewitness identification.

(Expert testimony on implantation of false memories

in repressed-memory cases would be another exam-

ple.) Courts rarely raise the credibility objection to this

evidence for a couple of reasons. First, a principled

line can be drawn between credibility and accuracy,

and the latter seems less invasive of the jury function.

Second, eyewitness experts usually do not testify

regarding the accuracy of a particular witness but,

instead, only to the general factors that might interfere

with eyewitness accuracy. Again, this form of testi-

mony is less invasive. Nonetheless, courts have not

been enthusiastic admirers of psychological expertise

on the unreliability of eyewitnesses. Objections to this

sort of expert testimony typically involve the question

whether the information is “beyond the ken” of the

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